Dental impression material and method



DENTAL IMPRESSION MATERIAL AND METHOD OF MAKING SAME John A. Cornell, Ardmore, and Harry E. Kemmler, Elkins Park, Pa., assignors to H. D. Justi & Son, Inc, Philadelphia, Pa., a corporation of Pennsylvania N Drawing. Application April 23, 1952, Serial No. 283,984

12 Claims. (Cl. 106-385) This invention relates to impression materials for taking impressions, pertaining particularly to materials for taking impressions of parts of human beings, such as of the mouth for dental purposes, and of the outer ear for aural purposes, and method of making same.

There have been many efforts in the past to provide impression materials having the desired attributes to be pointed out, but so far as known each has been possessed of certain disadvantageous features militating against completely satisfactory use. Such previous materials in general fall into two main groups.

One group is of the hydrocolloid type. The hydrocolloids are thermoplastic materials, which are well known to the dental profession. As these must be heated to acquire the receptiveness to deformation necessary for forming the impression, and must be chilled in order to retain the impression, they are unpleasant to the patient and difiicult to manipulate by the dentist. The operating temperatures must be accurately controlled, the use of the hydrocolloids involves extensive manipulation and bulky cooling equipment, and with undercuts or the like in the model, as is inherent in teeth, are found'to be more or less inaccurate in withdrawing the formed impression.

The other group of impression materials are the alginate materials. This group is more satisfactory from the equipment and handling point of view, and also from the viewpoint of the patient. The processing of materials of this group involves the gelling of a soluble potassium or sodium alginate, by the gradual dissolving of a slightly soluble calcium salt, producing calcium ions which form a gel with alginate. The primary disadvantage of this type of material is, that although the formed gel is elastic, in the sense that it tends to return by its own resilience to its acquired or imparted form after deformation as in removal from contact with the teeth, it is not possessed of adequate dimensional stability and the strength thereof is frequently insufficient or inadequate to prevent the breaking off of thin undercuts or ridges.

The foregoing disadvantages of hydrocolloids and alginates are obviated by the present invention and the basic requirements of the setting material are satisfied, the requirements being that the product set rapidly in the mouth to a gel of adequate dimensional stability sufficient to preclude tearing, and with high elasticity, from which very accurate reproductions of the structure of the mouth can be made for dental purposes. In accomplishing these objectives it will be understood further that it is essential that the material have a controlled time of gelation, so that the material will not set while mixing and preparing the tray, and yet will set in a short period of time so as to effect minimal discomfort to the patient.

It is among the objects of this invention to improve impression materials; to provide impression materials of high elasticity with dimensional stability; to provide impression materials of closely controlled time of setting from the initial mixing to the final set; to provide economical and efiicient impression materials for dental and aural purposes with minimal reaction to human tissues; and to provide other objects and advantages as will'become more apparent as the description proceeds.

In carrying out the invention, a modified natural polymeric material which contains in it free hydroxyl groups and free carboxyl groups, carboxy methyl cellulose, is utilized as a gelling material. Carboxy methyl cellulose available as a commercial material and produced by the carboxylation of cellulose is available in various grades based upon the molecular weight distribution of the cell.

component and upon the degree of substitution of carboxyl groups, this degree of substitution designated com:

monly as the number of carboxyl groups for each repeating anhydroglucose unit of the cellulose. The commercial carboxy methyl cellulose in the form of the alkali metal salt or ammonium salt or in the commercial neutral preparation is water soluble while in the acid form it is sparingly soluble in water. The water soluble form of carboxy methyl cellulose is the form used in accordance with the invention. The degree of substitution of the carboxyl groups is preferably low in order to obtain a relatively high molecular weight of the cellulose, thereby producing a stronger gel.

Carboxy methyl cellulose in a water solution will precipitate to form a gel with certain monovalent or divalent ions, such as copper (Cu++), lead (Pb++), silver (Ag+), and tin (Sn It will precipitate to form a gel with trivalent ions. The method of the invention further consists in controlling the rate of reaction of a trivalent ion with carboxy methyl cellulose to form a gel therewith by the controlled oxidation of a divalent ion, other than the divalent ions which precipitate carboxy methyl cellulose and to provide precipitating trivalent ions of the same metal as the divalent ion. This control is difiicult. If, for example, ferric ion (Fe+++) in the form of ferric chloride (Fecls) in a solution is added to carboxy methyl cellulose or CMC an immediate gel is formed, producing an uneven mixture containing lumps, loose unreacted areas and agglomerated particles. Similar undesirable results are obtained with the gel produced by the addition of freely available aluminum ion (Al+++).

In order to control the rate of gelation, it has been discovered one may slow the reaction cycle by producing ferric ion (Fe+++) at a controlled rate by oxidizing at a controlled rate ferrous ions in the reservoir of ferrous ions present in the solutions and thus there is obtained a desirable slowing down of the rate of gelation. Accordingly, a procedure for this purpose is by use of an oxidation-reduction system.

lllustratively, this controlled production of ferric ion (Fe+++) may be produced by employing a highly water soluble ferrous salt or by using a slightly soluble ferrous salt, such as ferrous oxalate (FeCzOi) in Water. The slightly soluble ferrous salt slowly dissolves and is oxidized to the desired valence by the oxidizing agent. If the highly soluble ferrous salt is used, the control of the rate of ferric ion formation is the result of the control of the rate of addition of the oxidizing agent. The ferric ion then reacts with the carboxy methyl cellulose to form a gel.

The rate of reaction, i.e. the formation of the gel can be'ch anged by any of four factors. (1) By changing the type and rate of solubility of the ferrous salt, as between, say, ferrous oxalate with loW solubility, and, say, ferrous chloride, with high solubility. (2) The pH of the solution can be changed by suitable buffers, i.e. the salts of weak acids and strong bases, or the salts of strong acids and Weak bases. Ionic strength of salt has an effect on reaction time. (3) The characteristics of the oxidizing agent selected, chosen, for instance, from a group comprising illustratively: chromic acid; potassium permanganate; potassium dichromate; potassium perchlorate;

Patented Jan. 31, 1956' weight.

Percent l. CMC 4.009 2. Ferrous oxalate 0.501 3. Starch 4.009 4. Cotton flock 1.002 5. Sodium citrate 0.301 6. Water 80.178

In the foregoing example the carboxy methyl cellulose, molecular weight 240, provides a molecular equivalent on the basis of about 1 carboxy group per glucose unit of about .0167. Ferrous oxalate, molecular weight 179.9, in the amount used above, provides a molecular equivalent of about .0029 on the basis of the combination of the metal ion with CMC, it is seen that the ferrous salt is about 17% of the available carboxy groups on this assumed molecular equivalent basis in the CMC.

Items 3 and 4 represent illustrative fillers and can be replaced by others as are well known in the art, the amount of filler in the example is at least as large as and somewhat larger than the amount of CMC employed.

Item 5 is an illustrative buffer, of many that may he used, as will be understood.

While it will be recognized that the percentages of solution phase one are less than 100%, the remaining percentage is provided by phase two, the oxidizing solution.

Illustratively, phase two comprises:

Percent Chromic acid 0.500 Water 9.500

Chromic acid having a molecular weight of 118 and used in an amount of 0.5% provides a molecular equivalent of .013, an excess over the molecular equivalent of ferrous oxalate, the entire amount being capable of bringing about the oxidation of the ferrous ion to the ferric Mixture of phases one and two provides a mass of time and degree-controlled gelation, adequate for proper mixing of the mass, application thereto to a support, insertion in the patients mouth, forming the mixture about the teeth, setting up the mixture into an elastic gel of substantial dimensional stability, and removing the formed gel as an integral imparted form-retaining mass.

It will be seen that although the two phase composition above described is highly useful, the chromic acid has a fast reaction and must be kept separated from the first phase until the time of use. It will be understood that from the standpoint of the user, it would be more satisfactory to have a given mass, such as a mass of powder, which requires merely the addition of water to establish the same beneficial controlled gelation as has been described. It is a further feature of this invention to provide such composition for solution in water. In carrying out the invention according to this portion thereof, in a one phase mixture, we are able to secure desir- 4 able results according to two types of procedures and materials.

According to the first procedure, a dry powder or like mass is formed, comprising polymeric material containing hydroxyl groups and carboxyl groups as discussed, a soluble salt of a divalent ion which is oxidizable to the trivalent ion, a filler, a buffer, and an oxidizing agent, the reaction of which cannot occur in the dry state. Illustratively, in this case, the chromic acid of phase two of the two phase system above is replaced by an oxidizing agent as potassium chlorate. This mass reacts to form a gel upon the addition of water.

What is claimed is:

1. A method of producing a water-containing gel from an aqueous solution of carboxymethyl cellulose containing a carboxyl group in recurring anhydroglucose units thereof, comprising incorporating in said solution an amount of ferrous ions in the form of a ferrous salt which is substantially less than the molecular equivalent for reacting with the carboxyl group and thereafter treating the solution with an oxidizing agent for said ferrous ions to form ferric ions which precipitate said carboxymethyl cellulose thereby forming a gel.

2. A method as claimed in claim 1 wherein said oxidizing agent is a water-soluble chromium salt.

3. A method as claimed in claim 1 wherein said oxidizing agent is a water soluble perchlorate salt.

4. A method as claimed in claim 1 wherein said oxidizing agent is a water-soluble chromic acid.

5. A method as claimed in claim 1 wherein said ferrous salt is ferrous oxalate which is slowly soluble in said solution.

6. A method as claimed in claim 1 wherein said ferrous salt is ferrous chloride which is very soluble in said solution.

7. An impression material consisting essentially on a dry weight basis of about 4 parts by weight of carboxymethyl cellulose containing a carboxyl group in recurring anhydroglucose units thereof, about 5 parts by weight of an inert filler, about /2 part of a ferrous salt to produce ferrous ions when dissolved in water which is substantially less than the molecular equivalent for reacting with the carboxyl group and about /2 part of an oxidizing agent for said ferrous salt to convert ferrous ions into ferric ions when dissolved in water.

8. A dental impression composition as claimed in claim 7 dissolved in about 9 parts of water for 1 part of the dry mixture of said material.

9. A dental impression material as claimed in claim 7 wherein said oxidizing agent is a water-soluble perchlorate and said ferrous salt is ferrous chloride.

10. A dental impression material as claimed in claim 7 wherein said oxidizing agent is a water-soluble chromium salt and said ferrous salt is ferrous oxalate.

11. A dental impression material as claimed in claim 7 wherein said oxidizing agent is chromic acid and said ferrous salt is ferrous oxalate.

12. A dental impression material as claimed in claim 7 wherein said oxidizing agent is a water-soluble perchlorate and said ferrous salt is ferrous oxalate.

References Cited in the file of this patent UNITED STATES PATENTS 2,520,805 Joy Aug. 29, 1950 

7. AN IMPRESSION MATERIAL CONSISTING ESSENTIALLY ON A DRY WEIGHT BASIS OF ABOUT 4 PARTS BY WEIGHT OF CARBOXYMETHYL CELLULOSE CONTAINING A CARBOXYL GROUP IN RECURRING ANHYDROGLUCOSE UNITS THEREOF, ABOUT 5 PARTS BY WEIGHT FERROUS IONS WHEN DISSOLVED IN WATER WHICH IS SUBSTANTIALLY LESS THAN THE MOLECULAR EQUIVALENT FOR REACTING WITH THE CARBOXYL GROUP AND ABOUT 1/2 PART OF AN OXIDIZING AGENT FOR SAID FERROUS SALT TO CONVERT FERROUS IONS INTO FERRIC IONS WHEN DISSOLVED IN WATER. 